Thread Content
The performance grades of bolts used for structural connections include more than 10 grades such as 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, and 12.9. Bolts of grade 8.8 and above are made of low-carbon alloy steel or medium-carbon steel and have undergone heat treatment (quenching and tempering); they are commonly referred to as high-strength bolts, while the rest are known as ordinary bolts. The bolt performance grade designation consists of two digits, which represent the nominal tensile strength value of the bolt material and the yield strength ratio, respectively. For example, a bolt with a performance grade of 4.6 means that: 1. The nominal tensile strength of the bolt material is at the 400 MPa level ； 2. The yield-to-tensile strength ratio of the bolt material is 0.6 ； 3. The bolt material has a nominal yield strength of 400×0.6=240 MPa, belonging to grade 10.9 high-strength bolts; after heat treatment, this material can achieve: 1. A nominal tensile strength of 1000 MPa for the bolt material ； 2. The yield-to-tensile strength ratio of the bolt material is 0.9 ； 3. The nominal yield strength of the bolt material is 1000×0.9=900 MPa. The performance grade of bolts represents a standard used internationally; bolts with the same performance grade have identical properties, regardless of their material or place of origin. In design, only the performance grade needs to be considered. The strength grades of 8.8 grade and 10.9 grade refer to the shear stress resistance levels of bolts, which are 8.8 GPa and 10.9 GPa respectively. For a grade of 8.8, the nominal tensile strength is 800 N/mm2 and the nominal yield strength is 640 N/mm2. Generally, bolts have their strength indicated as “X.Y”; where X*100 represents the bolt’s tensile strength, and X*100*(Y/10) represents its yield strength (as per the standard specification: yield strength/tensile strength = Y/10). For example, in the case of a 4.8 grade bolt, its tensile strength is 400 MPa, while its yield strength is 400*8/10 = 320 MPa =================